This invention relates generally to rolling bearings, and in particular to an elastomeric spring for a bearing assembly in an electric motor which inhibits fretting damage and extends the life of the bearing.
A substantial concern in machinery is fretting, which results from slight, micro-mechanical relative movements between tightly fitting parts which superficially appear immobile with respect to each other. Fretting is a combination of corrosion and abrasion which is frequently observed on equipment with moving or vibrating parts. If it continues, fretting can result in pitting and galling of contact surfaces, vibration, increased stress concentrations, and reduced life.
In an electric motor, fretting is a primary cause of bearing failure. As known to those skilled in the art, a motor has a rotatable assembly, or rotor, mounted within a hollow core of a stationary assembly, or stator, which holds windings of insulated wire. The motor transforms electrical energy into mechanical torque when the windings of the stator are energized with electrical current and interact with the rotor as it rotates. A rotatable shaft extends through the motor and is mounted on bearings located at front and back endshields on opposite longitudinal ends. A ball bearing, for example, has a cylindric outer surface and is received in a bore of the respective endshield. Unfortunately, the outer surface of the bearing is subject to rotate relative to the bore in which it is received. The bearing is typically sized with a clearance fit in the bore for ease of assembly and for avoiding radial loads on the bearing, but that decreases frictional resistance along the bearing's cylindric outer side and tends to facilitate relative movement. Even a bearing which is sized for a tight fit tends to become loose when ambient temperature varies because of differential thermal expansions between the bearing and endshield. As a result, slight relative movements occur between outer bearing surface and bore, which produce fretting damage.
Frequently, an annular-shaped steel washer is placed between the stationary end face of one bearing and a bottom shoulder of the corresponding bore to accommodate tolerance build-up and design clearances. The washer is not perfectly flat but has a “wavy” circumferential contour with several (e.g., three) high points which smoothly transition to intervening low points. When compressed between the bearing and end of the bore, the washer functions as a compression spring and applies reaction force to the bearing in the axial direction. The spring washer beneficially pushes the shaft and entire rotor assembly to a stable position to “preload” the rotor and reduce axial movement while accommodating component tolerances. Unfortunately, the washer does not provide significant resistance to rotation of the bearing. That limitation arises for at least two reasons. First, the contour of the washer provides only a small area of contact between the washer and the end face of the bearing, specifically at the three high points. Second, surface adhesion (i.e., friction coefficient) at the area of contact is relatively low because the materials in contact are both metallic. Thus, the bearing easily slips (rotates) on the spring washer.
As fretting continues, the bearing eventually moves in a radial direction, known to those skilled in the art as bearing creep, which causes the rotor to strike the stator. The entire motor must then be replaced. Several previous approaches for preventing rotation of the bearing have failed or required an additional insert or adhesive along the outer sides of the bearing, which increases complexity, degrades reliability, or makes assembly more difficult.